MRI is an imaging method which magnetically excites nuclear spin of an object set in a static magnetic field with an RF (radio frequency) signal having the Larmor frequency and reconstructs an image based on MR (magnetic resonance) signals generated due to the excitation. That is, in an MRI apparatus, an RF pulse having the resonant frequency proportional to an intensity of the static magnetic field is applied to an object in order to generate the nuclear magnetic resonance phenomenon.
In an MRI apparatus, an excitation for slice selection is often performed in order to select a slice in which MR signals generate. The excitation for slice selection is carried out by applying a gradient magnetic field for SS (slice selection). When an SS gradient magnetic field is applied, the resonant frequency of MR signals actually differs from the resonant frequency which depended on only a static magnetic field. However, magnetic spins can be considered to be excited at an on-resonance, i.e., at a resonance state, regardless of an existence of an excitation for slice selection.
On the other hand, the acquisition method of MR signals with an application of an off-resonance RF signal is known. In this method, an off-resonance RF pulse having a carrier frequency which is different from the resonant frequency by about several kHz is applied to an object. As typical examples of an imaging method using an off-resonance RF pulse, the MTC (magnetization transfer contrast) method, the measuring method of an RF magnetic field (B1) using Bloch-Siegert shift, and the CEST (chemical exchange saturation transfer) method are known.
In the MTC method, an off-resonance RF pulse is applied in advance of a gradient magnetic field pulse for SS in a pulse sequence under the FE (field echo) method, which is also referred to as the GE (gradient echo) method. Moreover, a gradient magnetic field spoiler pulse for spoiling transverse magnetization signals is applied after the application of the off-resonance RF pulse.
The off-resonance RF pulse applied in the MTC method is referred to as the MT (magnetization transfer) pulse. The MT pulse saturates magnetizations of giant molecules in tissues and the protons in bound water to generate an MT (magnetization transfer) to protons in free water. Since this MT effect has a dependency on tissue, using the MT effect makes it possible to make a diagnostic image clear.
Moreover, the off-resonance RF pulse is also used for an acquisition of a distribution image of B1 intensity. The measurement of B1 intensity using the off-resonance RF pulse uses the Bloch-Siegert shift. The Bloch-Siegert shift is the shift in the resonant frequency which arises with a constant rate when MR signals are acquired with applying an RF signal having a frequency close to the observed frequency. Specifically, a phase shift of MR signals, which occurs by an excitation with the on-resonance RF pulse and a subsequent further excitation with the off-resonance RF pulse, is used for imaging.
Generally, when the intensity of the static magnetic field becomes large, the B1 intensity becomes non-uniform inside a living body due to an electric loss and a dielectric loss in the living body. For this reason, there is a possibility that the uniformity in images may deteriorate. Accordingly, the measurement of the B1 intensity is desired for an improvement of ununiformity in the B1 intensity and an estimation of the B1 intensity. Although various techniques are proposed as measuring methods of the B1 intensity, the measuring method of the B1 intensity using an off-resonance RF pulse is one of the few methods which can encode the B1 intensity to the phase of MR signals.
In magnetic resonance imaging (MRI) with application of an off-resonance RF pulse, a change in contrast and artifact due to a phase shift are problems. For this reason, it is desired to improve contrast and image quality much more in MRI with an application of an off-resonance RF pulse.
It is an object of the present invention to provide a magnetic resonance imaging apparatus and a magnetic resonance imaging method which can acquire an MR image having a more improved contrast and image quality applying an off-resonance RF pulse.